Saw runs champ with Motomix but terrible with mixed fuel....

[trboxman]

more like rancid fry oil to my nose, but then again I don't have the smell faculties that I used to have...

 

[astnmacgto]

Given that description I'd speculate that a stiff diaphragm would push motomix better than it would pump mix at the same mix ratio, but that would be a hair split difference...

In theory you would be correct but in real world testing are the viscosities different enough that you would ever know? Makes me curious, but at the same instance if one saw in my fleet only ran well on MotoMix I would just use that and not worry about it, to each his own

 

[bmwpowere36m3]

Anybody else think the exhaust of a saw running motomix smells a bit like burning plastic?

Stihl Ultra....

 

[row.man]

run avgas, there is nothing nicer to smell that 2 cycle mixed with 100LL

 

[Rockjock]

If the saw runs badly, tuning is not OK. The tuning needs to be adjusted to the operational conditions of the saw (carburetor performance, air intake, exhaust performance, etc.), hence the word "tuning", and one of the factors there is the quality of the fuel. Pump fuel has a different viscosity when compared to Motomix, so when changing from Motomix to pump mix with the engine adjusted for Motomix the carburetor will need to be re-adjusted to compensate.
So, tuning was correct for Motomix but definitely not correct for pump mix. Some saws (carburetors) can handle both without adjustment but probably do not run at 100% with either...

[I edited this one - one always should think and then type ]

It is not only viscosity but also the way the fuel burns under compression, the amount of air needed for combustion and the temperatures produced. Generally, Motomix and the like burn hotter (leaner) than pump fuel...

ok so let me clarify. The saw runs very well on motomix, but not on mixed fuel. The tuning is correct. Adjusted every which way from Sunday and it will not run correctly. Thus the interesting problem. I have seen this one time before and it too was a mystery but somewhat explainable as this saw only ever ran motomix. The carb kit went in today and the saw is running like it should. kit looks fine, but it a little " softer " than normal.. now to find some screws for a ms880 carb.. and then I can post it and get many " You Suck " messages

 

[7sleeper]

You mentioned "carb rebuilt previously", was it rebuilt with OEM parts or aftermarket? Further it would have been interesting to me to see how the saw would run after ~ half a tank full? Maybe it would only require a little "run in time" with the new fuel?

7

 

[Rockjock]

You mentioned "carb rebuilt previously", was it rebuilt with OEM parts or aftermarket? Further it would have been interesting to me to see how the saw would run after ~ half a tank full? Maybe it would only require a little "run in time" with the new fuel?

7

OEM parts only, I work at a dealer aftermarket parts are verboten! We would tune the saw and it ran poorly, once it ran the moto mix it would run smooth and quite powerful. I add it to the list of life mysteries!

 

[earlthegoat2]

I'm curious how the tune could have ever been "correct" for mix gas if you never got decent results from tuning it for the mix. If I can't tune a running problem out of a saw then I look for problems elsewhere. Tuning is never correct until the correct overall results are achieved.

"We would tune the saw and it ran poorly..." Tuning the saw does not just mean turning the carb screws.

Glad you got the problem worked out though.